Nearly all of the overhead in IPsec processing is in the encryption
and authentication of packets. Our
performance document discusses these overheads.

Beside those overheads, the cost of managing additional tunnels is
trivial. Whether your gateway supports one tunnel or ten just does not
matter. A hundred might be a problem; there is a
section on this in the performance document.

So, in nearly all cases, if using multiple tunnels gives you a
reasonable way to describe what you need to do, you should describe it
that way in your configuration files.

For example, one user recently asked on a mailing list about this
network configuration:

netA---gwA---gwB---netB
|----netC
netA and B are secured netC not.
netA and gwA can not access netC

The user had constructed only one tunnel, netA to netB, and wanted to
know how to use ip-route to get netC packets into it. This is entirely
unnecessary. One of the replies was:

The simplest way and indeed the right way to
solve this problem is to set up two connections:
leftsubnet=NetA
left=gwA
right=gwB
rightsubnet=NetB
and
leftsubnet=NetA
left=gwA
right=gwB
rightsubnet=NetC

This would still be correct even if we added nets D, E, F, ... to the
above diagram and needed twenty tunnels.

Of course another possibility would be to just use one tunnel, with a
subnet mask that includes both netB and netC (or B, C, D, ...). See
next section.

In general, you can construct as many tunnels as you need. Networks
like netC in this example that do not connect directly to the gateway
are fine, as long as the gateway can route to them.

The number of tunnels can become an issue if it reaches 50 or so.
This is discussed in the performance
document. Look there for information on supporting hundreds of Road
Warriors from one gateway.

If you find yourself with too many tunnels for some reason like
having eight subnets at one location and nine at another so you end up
with 9*8=72 tunnels, read the next section here.

The subnets used in leftsubnet and rightsubnet
can be of any size that fits your needs, and they need not correspond
to physical networks.

You adjust the size by changing the
subnet mask, the number after the slash in the subnet description.
For example

in 192.168.100.0/24 the /24 mask says 24 bits are used to designate
the network. This leave 8 bits to label machines. This subnet has 256
addresses. .0 and .255 are reserved, so it can have 254 machines.

A subnet with a /23 mask would be twice as large, 512 addresses.

A subnet with a /25 mask would be half the size, 128 addresses.

/0 is the whole Internet

/32 is a single machine

As an example of using these in connection descriptions, suppose your
company's head office has four physical networks using the address
ranges:

192.168.100.0/24

development

192.168.101.0/24

production

192.168.102.0/24

marketing

192.168.103.0/24

administration

You can use exactly those subnets in your connection descriptions, or
use larger subnets to grant broad access if required:

leftsubnet=192.168.100.0/24

remote hosts can access only development

leftsubnet=192.168.100.0/23

remote hosts can access development or production

leftsubnet=192.168.102.0/23

remote hosts can access marketing or administration

leftsubnet=192.168.100.0/22

remote hosts can access any of the four departments

or use smaller subnets to restrict access:

leftsubnet=192.168.103.0/24

remote hosts can access any machine in administration

leftsubnet=192.168.103.64/28

remote hosts can access only certain machines in administration.

leftsubnet=192.168.103.42/32

remote hosts can access only one particular machine in
administration

To be exact, 192.68.103.64/28 means all addresses whose top 28 bits
match 192.168.103.64. There are 16 of these because there are 16
possibilities for the remainingg 4 bits. Their addresses are
192.168.103.64 to 192.168.103.79.

Each connection description can use a different subnet if required.

It is possible to use all the examples above on the same FreeS/WAN
gateway, each in a different connection description, perhaps for
different classes of user or for different remote offices.

It is also possible to have multiple tunnels using different
leftsubnet descriptions with the same right. For
example, when the marketing manager is on the road he or she might have
access to:

leftsubnet=192.168.102.0/24

all machines in marketing

192.168.101.32/29

some machines in production

leftsubnet=192.168.103.42/32

one particular machine in administration

This takes three tunnels, but tunnels are cheap. If the laptop is set
up to build all three tunnels automatically, then he or she can access
all these machines concurrently, perhaps from different windows.

This can be described as a special case of the general
subnet-to-subnet connection. The subnet on the right is 0.0.0.0/0, the
whole Internet.

West (the home gateway) can have its firewall rules set up so that
only IPsec packets to East are allowed out. It will then behave as if
its only connection to the world was a wire to East.

When machines on the home network need to reach the Internet, they do
so via the tunnel, East and the corporate firewall. From the viewpoint
of the Internet (perhaps of some EvilDoer trying to break in!), those
home office machines are behind the firewall and protected by it.

use keys automatically generated by the Pluto key negotiation
daemon. The key negotiation protocol, IKE
, must authenticate the other system. (It is vulnerable to a
man-in-the-middle attack if used without authentication.) We
currently support two authentication methods:

A third method, using RSA keys embedded in
X.509 certtificates, is provided by user
patches.

Manually keyed connections provide
weaker security than automatically keyed
connections. An opponent who reads ipsec.secrets(5) gets your
encryption key and can read all data encrypted by it. If he or she has
an archive of old messages, all of them back to your last key change
are also readable.

With automatically-(re)-keyed connections, an opponent who reads
ipsec.secrets(5) gets the key used to authenticate your system in IKE
-- the shared secret or your private key, depending what authentication
mechanism is in use. However, he or she does not automatically gain
access to any encryption keys or any data.

An attacker who has your authentication key can mount a
man-in-the-middle attack and, if that succeeds, he or she will get
encryption keys and data. This is a serious danger, but it is better
than having the attacker read everyting as soon as he or she breaks
into ipsec.secrets(5).. Moreover, the keys change often so an opponent
who gets one key does not get a large amount of data. To read all your
data, he or she would have to do a man-in-the-middle attack at every
key change.

We discuss using manual keying in production
below, but this is not recommended except in special
circumstances, such as needing to communicate with some implementation
that offers no auto-keyed mode compatible with FreeS/WAN.

Manual keying may also be useful for testing. There is some
discussion of this in our FAQ.

The IKE protocol which Pluto uses to negotiate connections between
gateways must use some form of authentication of peers. A gateway must
know who it is talking to before it can create a secure connection. We
support two basic methods for this authentication:

Putting public keys in DNS allows us to support
opportunistic encryption. Any two FreeS/WAN gateways can provide
secure communication, without either of them having any preset
information about the other.

Authentication with a public key
method such as RSA has some important
advantages over using shared secrets.

no problem of secure transmission of secrets

A shared secret must be shared, so you have the problem of
transmitting it securely to the other party. If you get this wrong, you
have no security.

With a public key technique, you transmit only your public key. The
system is designed to ensure that it does not matter if an enemy
obtains public keys. The private key never leaves your machine.

easier management

Suppose you have 20 branch offices all connecting to one gateway at
head office, and all using shared secrets. Then the head office admin
has 20 secrets to manage. Each of them must be kept secret not only
from outsiders, but also from 19 of the branch office admins. The
branch office admins have only one secret each to manage.

If the branch offices need to talk to each other, this becomes
problematic. You need another 20*19/2 = 190 secrets for
branch-to-branch communication, each known to exactly two branches. Now
all the branch admins have the headache of handling 20 keys, each
shared with exactly one other branch or with head office.

For larger numbers of branches, the number of connections and secrets
increases quadratically and managing them becomes a nightmare. A
1000-gateway fully connected network needs 499,500 secrets, each known
to exactly two players. There are ways to reduce this problem, for
example by introducing a central key server, but these involve
additional communication overheads, more administrative work, and new
threats that must be carefully guarded against.

With public key techniques, the only thing you have to keep
secret is your private key, and you keep that secret from everyone
.

As network size increaes, the number of public keys used increases
linearly with the number of nodes. This still requires careful
administration in large applications, but is nothing like the disaster
of a quadratic increase. On a 1000-gateway network, you have 1000
private keys, each of which must be kept secure on one machine, and
1000 public keys which must be distributed. This is not a trivial
problem, but it is manageable.

does not require fixed IP addresses

When shared secrets are used in IPsec, the responder must be able to
tell which secret to use by looking at the IP address on the incoming
packets. When the other parties do not have a fixed IP address to be
identified by (for example, on nearly all dialup ISP connections and
many cable or ADSL links), this does not work well -- all must share
the same secret!

When RSA authentication is in use, the initiator can identify itself
by name before the key must be determined. The responder then checks
that the message is signed with the public key corresponding to that
name.

There is also a disadvantage:

your private key is a single point of attack, extremely valuable to
an enemy

with shared secrets, an attacker who steals your ipsec.secrets file
can impersonate you or try
man-in-the-middle attacks, but can only attack connections
described in that file

an attacker who steals your private key gains the chance to attack
not only existing connections but also any future connections
created using that key

This is partly counterbalanced by the fact that the key is never
transmitted and remains under your control at all times. It is likely
necessary, however, to take account of this in setting security policy.
For example, you should change gateway keys when an administrator
leaves the company, and should change them periodically in any case.

Overall, public key methods are more secure, more easily
managed and more flexible. We recommend that they be used for
all connections, unless there is a compelling reason to do otherwise.

Generally, public key methods are preferred for reasons given above,
but shared secrets can be used with no loss of security, just more work
and perhaps more need to take precautions.

What I call "shared secrets" are sometimes also called "pre-shared
keys". They are used only for for authentication, never for encryption.
Calling them "pre-shared keys" has confused some users into thinking
they were encryption keys, so I prefer to avoid the term..

If you are interoperating with another IPsec implementation, you may
find its documentation calling them "passphrases".

make the secrets long and unguessable. Since they need not be
remembered by humans, very long ugly strings may be used. We suggest
using our ipsec_ranbits(8)
utility to generate long (128 bits or more) random strings.

transmit secrets securely. You have to share them with other
systems, but you lose if they are intercepted and used against you. Use PGP, SSH,
hand delivery of a floppy disk which is then destroyed, or some other
trustworthy method to deliver them.

limit sharing of secrets. Alice, Bob, Carol and Dave may all talk to
each other, but only Alice and Bob should know the secret for an
Alice-Bob link.

do not share private keys. The private key for RSA
authentication of your system is stored in
ipsec.secrets(5), but it is a different class of secret from the
pre-shared keys used for the "shared secret" authentication. No-one but
you should have the RSA private key.

Each line has the IP addresses of the two gateways plus the secret.
It should look something like this:

PSK indicates the use of a pre-s
hared key. The quotes and the whitespace shown are
required.

You can use any character string as your secret. For security, it
should be both long and extremely hard to guess. We provide a utility
to generate such strings,
ipsec_ranbits(8).

You want the same secret on the two gateways used, so you create a
line with that secret and the two gateway IP addresses. The
installation process supplies an example secret, useful only
for testing. You must change it for production use.

You must deliver this file, or the relevant part of it, to the other
gateway machine by some secure means. Don't just
FTP or mail the file! It is vital that the secrets in it remain
secret. An attacker who knew those could easily have all the data
on your "secure" connection.

This file must be owned by root and should have permissions
rw-------.

You can use a shared secret to support a single road warrior
connecting to your gateway, and this is a reasonable thing to do in
some circumstances. Public key methods have advantages, discussed
above, but they are not critical in this case.

For more than one road warrior, shared secrets are not
recommended. If shared secrets are used, then when the
responder needs to look up the secret, all it knows about the sender is
an IP address. This is fine if the sender is at a fixed IP address
specified in the config file. It is also fine if only one road warrior
uses the wildcard 0.0.0.0 address. However, if you have more
than one road warrior using shared secret authentication, then they
must all use that wildcard and therefore all road warriors
using PSK autentication must use the same secret. Obviously,
this is insecure.

For multiple road warriors, use public key authentication.
Each roadwarrior can then have its own identity (our leftid=
or rightid= parameters), its own public/private key pair,
and its own secure connection.

However, it is possible to use manual keying in production if that is
what you want to do. This might be necessary, for example, in order to
interoperate with some device that either does not provide automatic
keying or provides it in some version we cannot talk to.

Note that with manual keying all security rests with the keys
. If an adversary acquires your keys, you've had it. He or she can read
everything ever sent with those keys, including old messages he or she
may have archived.

You need to be really paranoid about keys if you're
going to rely on manual keying for anything important.

keep keys in files with 600 permissions, owned by root

be extremely careful about security of your gateway systems. Anyone
who breaks into a gateway and gains root privileges can get all your
keys and read everything ever encrypted with those keys, both old
messages he has archived and any new ones you may send.

change keys regularly. This can be a considerable bother, (and
provides an excellent reason to consider automatic keying instead), but
it is absolutely essential for security. Consider a manually
keyed system in which you leave the same key in place for months:

an attacker can have a very large sample of text sent with that key
to work with. This makes various cryptographic attacks much more likely
to succeed.

The chances of the key being compromised in some non-cryptographic
manner -- a spy finds it on a discarded notepad, someone breaks into
your server or your building and steals it, a staff member is bribed,
tricked, seduced or coerced into revealing it, etc. -- also increase
over time.

a successful attacker can read everything ever sent with that key.
This makes any successful attack extremely damaging.

It is clear that you must change keys often to have any useful
security. The only question is how often.

don't edit files with keys in them when someone can look over your
shoulder

worry about network security; could someone get keys by snooping
packets on the LAN between your X desktop and the gateway?

lock up your backup tapes for the gateway system

... and so on

Linux FreeS/WAN provides some facilities to help with this. In
particular, it is good policy to keep keys in separate files
so you can edit configuration information in /etc/ipsec.conf without
exposing keys to "shoulder surfers" or network snoops. We support this
with the also= and include syntax in
ipsec.conf(5).

See the last example in our examples file. In
the /etc/ipsec.conf conn samplesep section, it has the line:

also=samplesep-keys

which tells the "ipsec manual" script to insert the configuration
description labelled "samplesep-keys" if it can find it. The
/etc/ipsec.conf file must also have a line such as:

include ipsec.*.conf

which tells it to read other files. One of those other files then
might contain the additional data:

The first line matches the label in the "also=" line, so the indented
lines are inserted. The net effect is exactly as if the inserted lines
had occurred in the original file in place of the "also=" line.

Variables set here are:

spi

A number needed by the manual keying code. Any 3-digit hex number
will do, but if you have more than one manual connection then
spi must be different for each connection.

esp

Options for ESP (Encapsulated
Security Payload), the usual IPsec encryption mode. Settings here are
for encryption using
triple DES and
authentication using MD5. Note that
encryption without authentication should not be used; it is insecure.

espenkey

Key for ESP encryption. Here, a 192-bit hex number for triple DES.

espauthkey

Key for ESP authentication. Here, a 128-bit hex number for MD5.

Note that the example keys we supply
are intended only for testing. For real use, you
should go to automatic keying. If that is not possible, create your own
keys for manual mode and keep them secret

Of course, any files containing keys must have 600
permissions and be owned by root.

If you connect in this way to multiple sites, we recommend that you
keep keys for each site in a separate file and adopt some naming
convention that lets you pick them all up with a single "include" line.
This minimizes the risk of losing several keys to one error or attack
and of accidentally giving another site admin keys which he or she has
no business knowing.

Also note that if you have multiple manually keyed connections on a
single machine, then the spi parameter must be different for
each one. Any 3-digit hex number is OK, provided they are different for
each connection. We reserve the range 0x100 to 0xfff for manual
connections. Pluto assigns SPIs from 0x1000 up for automatically keyed
connections.

If ipsec.conf(5) contains
keys for manual mode connections, then it too must have permissions
rw-------. We recommend instead that, if you must manual keying
in production, you keep the keys in separate files.

Note also that ipsec.conf
is installed with permissions rw-r--r--. If you plan to use
manually keyed connections for anything more than initial testing, you
must:

either change permissions to rw-------

or store keys separately in secure files and access them via include
statements in ipsec.conf.

We recommend the latter method for all but the simplest
configurations.

Note that any temporary files used must be kept
secure since they contain keys. That is the reason for the
umask command above. The temporary file should be deleted as soon as
you are done with it. You may also want to change the umask back to its
default value after you are finished working on keys.

The ranbits utility may pause for a few seconds if not enough entropy
is available immediately. See ipsec_ranbits(8) and random(4) for
details. You may wish to provide some activity to feed entropy into the
system. For example, you might move the mouse around, type random
characters, or do du /usr > /dev/null in the background.

You can tell the system to set up connections automatically at boot
time by putting suitable stuff in /etc/ipsec.conf on both systems. The
relevant section of the file is labelled by a line reading config
setup.

Tells KLIPS which interfaces to use. Up to four interfaces numbered
ipsec[0-3] are supported. Each interface can support an arbitrary
number of tunnels.

Note that for PPP, you give the ppp[0-9] device name here, not the
underlying device such as modem (or eth1 if you are using PPPoE).

interfaces=%defaultroute

Alternative setting, useful in simple cases. KLIPS will pick up both
its interface and the next hop information from the settings of the
Linux default route.

forwardcontrol=no

Normally "no". Set to "yes" if the IP forwarding option is disabled
in your network configuration. (This can be set as a kernel
configuration option or later. e.g. on Redhat, it's in
/etc/sysconfig/network and on SuSE you can adjust it with Yast.) Linux
FreeS/WAN will then enable forwarding when starting up and turn it off
when going down. This is used to ensure that no packets will be
forwarded before IPsec comes up and takes control.

syslog=daemon.error

Used in messages to the system logging daemon (syslogd) to specify
what type of software is sending the messages. If the settings are
"daemon.error" as in our example, then syslogd treats the messages as
error messages from a daemon.

Note that Pluto does not currently
pay attention to this variable. The variable controls setup messages
only.

Normally, leave empty as shown above for no debugging output.
Use "all" for maximum information.
See ipsec_klipsdebug(8) and ipsec_pluto(8) man page for other
options. Beware that if you set /etc/ipsec.conf to enable debug output,
your system's log files may get large quickly.

dumpdir=/safe/directory

Normally, programs started by ipsec setup don't crash. If they do,
by default, no core dump will be produced because such dumps would
contain secrets. If you find you need to debug such crashes, you can
set dumpdir to the name of a directory in which to collect the core
file.

manualstart=

List of manually keyed connections to be automatically started at
boot time. Useful for testing, but not for long term use. Connections
which are automatically started should also be automatically re-keyed.

pluto=yes

Whether to start Pluto when ipsec
startup is done.
This parameter is optional and defaults to "yes" if not present.

"yes" is strongly recommended for production use so that the keying
daemon (Pluto) will automatically re-key the connections regularly. The
ipsec-auto parameters ikelifetime, ipseclifetime and reykeywindow give
you control over frequency of rekeying.

plutowait=no

Controls whether Pluto waits for one tunnel to be established before
starting to negotiate the next. You might set this to "yes"

if your gateway is a very limited machine and you need to conserve
resources.

for debugging; the logs are clearer if only one connection is
brought up at a time

For a busy and resource-laden production gateway, you likely want "no"
so that connections are brought up in parallel and the whole process
takes less time.

The example assumes you are at the Reno office and will use IPsec to
Vancouver, New York City and Amsterdam.

Consider a pair of subnets, each with a security gateway, connected
via the Internet:

192.168.100.0/24 left subnet
|
192.168.100.1
North Gateway
101.101.101.101 left
|
101.101.101.1 left next hop
[Internet]
202.202.202.1 right next hop
|
202.202.202.202 right
South gateway
192.168.200.1
|
192.168.200.0/24 right subnet

Without these, neither gateway can do IPsec to the remote subnet.
There is no IPsec tunnel or eroute set up for the traffic.

In our example, with the non-routable 192.168.* addresses used,
packets would simply be discarded. In a different configuration, with
routable addresses for the remote subnet, they would be sent
unencrypted since there would be no IPsec eroute and there
would be a normal IP route.

This is required if you want the two gateways to speak IPsec to each
other.

This requires a lot of duplication of details. Judicious use of
also= and include can reduce this problem.

Note that, while FreeS/WAN supports all four tunnel types, not all
implementations do. In particular, some versions of Windows 2000 and
the freely downloadable version of PGP provide only "client"
functionality. You cannot use them as gateways with a subnet behind
them. To get that functionality, you must upgrade to Windows 2000
server or the commercially available PGP products.

It is also possible to use the new routing features in 2.2 and later
kernels to avoid most needs for multple tunnels. Here is one mailing
list message on the topic:

Subject: Re: linux-ipsec: IPSec packets not entering tunnel?
Date: Mon, 20 Nov 2000
From: Justin Guyett <jfg@sonicity.com>
On Mon, 20 Nov 2000, Claudia Schmeing wrote:
> Right Left
> "home" "office"
> 10.92.10.0/24 ---- 24.93.85.110 ========= 216.175.164.91 ---- 10.91.10.24/24
>
> I've created all four tunnels, and can ping to test each of them,
> *except* homegate-officenet.
I keep wondering why people create all four tunnels. Why not route
traffic generated from home to 10.91.10.24/24 out ipsec0 with iproute2?
And 99% of the time you don't need to access "office" directly, which
means you can eliminate all but the subnet<->subnet connection.

and FreeS/WAN technical lead Henry Spencer's comment:

> I keep wondering why people create all four tunnels. Why not route
> traffic generated from home to 10.91.10.24/24 out ipsec0 with iproute2?
This is feasible, given some iproute2 attention to source addresses, but
it isn't something we've documented yet... (partly because we're still
making some attempt to support 2.0.xx kernels, which can't do this, but
mostly because we haven't caught up with it yet).
> And 99% of the time you don't need to access "office" directly, which
> means you can eliminate all but the subnet<->subnet connection.
Correct in principle, but people will keep trying to ping to or from the
gateways during testing, and sometimes they want to run services on the
gateway machines too.

Full opportunism allows you to initiate and receive opportunistic
connections on your machine. The remaining instructions in this section
assume you have first set up full opportunism on your gateway using
these instructions. Both sets of instructions require mailing DNS
records to your ISP. You may wish to collect DNS records for both the
gateway and the subnet nodes before contacting your ISP.

FreeS/WAN 2.x ships with a built-in, automatically enabled OE
connection conn packetdefault which applies OE, if possible,
to all outbound traffic routed through the FreeS/WAN box. The
ipsec.conf(5) manual describes this connection in detail. While the
effect is much the same as private-or-clear, the
implementation is different: notably, it does not use policy groups.

If your buddy has some unused IP addresses, in his subnet far off at
the other side of the Internet, he can loan them to you... provided
that the connection between you and him is fast enough to carry all the
traffic between your machines and the rest of the Internet. In effect,
he "extrudes" a part of his address space over the network to you, with
your Internet traffic appearing to originate from behind his Internet
gateway.

As far as the Internet is concerned, your new extruded net is behind
your buddy's gateway. You route all your packets for the Internet at
large out his gateway, and receive return packets the same way. You
route your local packets locally.

Suppose your friend has a.b.c.0/24 and wants to give you
a.b.c.240/28. The initial situation is:

subnet gateway Internet
a.b.c.0/24 a.b.c.1 p.q.r.s

where anything from the Internet destined for any machine in a.b.c.0/24
is routed via p.q.r.s and that gateway knows what to do from there.

Of course it is quite normal for various smaller subnets to exist
behind your friend's gateway. For example, your friend's company might
have a.b.c.16/28=development, a.b.c.32/28=marketing and so on. The
Internet neither knows not cares about this; it just delivers packets
to the p.q.r.s and lets the gateway do whatever needs to be done from
there.

What we want to do is take a subnet, perhaps a.b.c.240/28, out of
your friend's physical location while still having your friend's
gateway route to it. As far as the Internet is concerned, you
remain behind that gateway.

In our example, the friend's security gateway is also his Internet
gateway, but this is not necessary. As long as all traffic from the
Internet to his addresses passes through the Internet gate, the
security gate could be a machine behind that. The IG would need to
route all traffic for the extruded subnet to the SG, and the SG could
handle the rest.

First, configure your subnet using the extruded addresses. Your
security gateway's interface to your subnet needs to have an extruded
address (possibly using a Linux virtual
interface, if it also has to have a different address). Your
gateway needs to have a route to the extruded subnet, pointing to that
interface. The other machines at your site need to have addresses in
that subnet, and default routes pointing to your gateway.

If any of your friend's machines need to talk to the extruded subnet,
they need to have a route for the extruded subnet, pointing at his
gateway.

Then set up an IPsec subnet-to-subnet tunnel between your gateway and
his, with your subnet specified as the extruded subnet, and his subnet
specified as "0.0.0.0/0".

If either side was doing firewalling for the extruded subnet before
the IPsec connection is set up, you'll need to poke holes in your
firewall to allow packets through.

And it all just works. Your SG routes traffic for 0.0.0.0/0 -- that
is, the whole Internet -- through the tunnel to his SG, which then
sends it onward as if it came from his subnet. When traffic for the
extruded subnet arrives at his SG, it gets sent through the tunnel to
your SG, which passes it to the right machine.

Remember that when ipsec_manual or ipsec_auto takes a connection
down, it does not undo the route it made for that connection.
This lets you take a connection down and bring up a new one, or a
modified version of the old one, without having to rebuild the route it
uses and without any risk of packets which should use IPsec
accidentally going out in the clear. Because the route always points
into KLIPS, the packets will always go there. Because KLIPS temporarily
has no idea what to do with them (no eroute for them), they will be
discarded.

If you do want to take the route down, this is what the
"unroute" operation in manual and auto is for. Just do an unroute after
doing the down.

Note that the route for a connection may have replaced an existing
non-IPsec route. Nothing in Linux FreeS/WAN will put that pre-IPsec
route back. If you need it back, you have to create it with the route
command.

Please note that Openswan now
features DHCP-over-IPsec, which is an alternate procedure for Virtual
IP address assignment.

Here is a mailing list message about another way to configure for
road warrior support:

Subject: Re: linux-ipsec: understanding the vpn
Date: Thu, 28 Oct 1999 10:43:22 -0400
From: Irving Reid <irving@nevex.com>
> local-------linux------internet------mobile
> LAN box user
> ...
> now when the mobile user connects to the linux box
> it is given a virtual IP address, i have configured it to
> be in the 10.x.x.x range. mobile user and linux box
> have a tunnel between them with these IP addresses.
> Uptil this all is fine.
If it is possible to configure your mobile client software *not* to
use a virtual IP address, that will make your life easier. It is easier
to configure FreeS/WAN to use the actual address the mobile user gets
from its ISP.
Unfortunately, some Windows clients don't let you choose.
> what i would like to know is that how does the mobile
> user communicate with other computers on the local
> LAN , of course with the vpn ?
> what IP address should the local LAN
> computers have ? I guess their default gateway
> should be the linux box ? and does the linux box need
> to be a 2 NIC card box or one is fine.
As someone else stated, yes, the Linux box would usually be the default
IP gateway for the local lan.
However...
If you mobile user has software that *must* use a virtual IP address,
the whole picture changes. Nobody has put much effort into getting
FreeS/WAN to play well in this environment, but here's a sketch of one
approach:
Local Lan 1.0.0.0/24
|
+- Linux FreeS/WAN 1.0.0.2
|
| 1.0.0.1
Router
| 2.0.0.1
|
Internet
|
| 3.0.0.1
Mobile User
Virtual Address: 1.0.0.3
Note that the Local Lan network (1.0.0.x) can be registered, routable
addresses.
Now, the Mobile User sets up an IPSec security association with the
Linux box (1.0.0.2); it should ESP encapsulate all traffic to the
network 1.0.0.x **EXCEPT** UDP port 500. 500/udp is required for the key
negotiation, which needs to work outside of the IPSec tunnel.
On the Linux side, there's a bunch of stuff you need to do by hand (for
now). FreeS/WAN should correctly handle setting up the IPSec SA and
routes, but I haven't tested it so this may not work...
The FreeS/WAN conn should look like:
conn mobile
right=1.0.0.2
rightsubnet=1.0.0.0/24
rightnexthop=1.0.0.1
left=0.0.0.0 # The infamous "road warrior"
leftsubnet=1.0.0.3/32
Note that the left subnet contains *only* the remote host's virtual
address.
Hopefully the routing table on the FreeS/WAN box ends up looking like
this:
% netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
1.0.0.0 0.0.0.0 255.255.255.0 U 1500 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 3584 0 0 lo
0.0.0.0 1.0.0.1 0.0.0.0 UG 1500 0 0 eth0
1.0.0.3 1.0.0.1 255.255.255.255 UG 1433 0 0 ipsec0
So, if anybody sends a packet for 1.0.0.3 to the Linux box, it should
get bundled up and sent through the tunnel. To get the packets for
1.0.0.3 to the Linux box in the first place, you need to use "proxy
ARP".
How this works is: when a host or router on the local Ethernet segment
wants to send a packet to 1.0.0.3, it sends out an Ethernet level
broadcast "ARP request". If 1.0.0.3 was on the local LAN, it would
reply, saying "send IP packets for 1.0.0.3 to my Ethernet address".
Instead, you need to set up the Linux box so that _it_ answers ARP
requests for 1.0.0.3, even though that isn't its IP address. That
convinces everyone else on the lan to send 1.0.0.3 packets to the Linux
box, where the usual FreeS/WAN processing and routing take over.
% arp -i eth0 -s 1.0.0.3 -D eth0 pub
This says, if you see an ARP request on interface eth0 asking for
1.0.0.3, respond with the Ethernet address of interface eth0.
Now, as I said at the very beginning, if it is *at all* possible to
configure your client *not* to use the virtual IP address, you can avoid
this whole mess.

The key issue here is that the config setup section of the
/etc/ipsec.conf configuration file lists the connection between
ipsecN and hardware interfaces, in the interfaces= variable.
At any time when ipsec setup start or ipsec setup
restart is run this variable must correspond to
the current real situation. More precisely, it must not
mention any hardware interfaces which don't currently exist. The
difficulty is that an ipsec setup start command is normally
run at boot time so interfaces that are not up then are mis-handled.

When the hardware *is* in place, IPsec has to be made aware of it.
Someday there may be a nice way to do this.

Right now, the way to do it is to fix the /etc/ipsec.conf
file appropriately, so interfaces reflects the new
situation, and then restart the IPsec subsystem. This does break any
existing IPsec connections.

If IPsec wasn't brought up at boot time, do

ipsec setup start

while if it was, do

ipsec setup restart

which won't be as quick.

If some of the hardware is to be taken out, before doing that, amend
the configuration file so interfaces no longer includes it, and do

One situation in which unencrypted tunnels might seem handy is when
otherwise some data would be encrypted twice. Alice wants a secure
tunnel from her machine to Bob's. Since she's behind one security
gateway and he's behind another, part of the tunnel that they build
passes through the tunnel that their site admins have built between the
gateways. All of Alice and Bob's messages are encrypted twice.

There are several ways to handle this.

Just accept the overhead of double encryption. The site admins might
choose this if any of the following apply:

policy says encrypt everything (usually, it should)

they don't entirely trust Alice and Bob (usually, if they don't have
to, they shouldn't)

if they don't feel the saved cycles are worth the time they'd need
to build a non-encrypted tunnel for Alice and Bob's packets (often,
they aren't)

Use a plain IP-in-IP tunnel. These are not well documented. A good
starting point is in the Linux kernel source tree, in
/usr/src/linux/drivers/net/README.tunnel.

Note that if Alice and Bob want end-to-end security, they must build
a tunnel end-to-end between their machines or use some other end-to-end
tool such as PGP or SSL that suits their data. The only question is
whether the admins build some special unencrypted tunnel for those
already-encrypted packets.